Anatomical Reappraisal of the Intersphincteric Space: A Histology-Based Reinterpretation of the Parks Classification of Anal Fistulas

Background: Anal fistula pathways have been traditionally described using the Parks classification; however, the microscopic nature of the “intersphincteric” space has never been characterized completely. Furthermore, the longitudinal muscle between the internal and external anal sphincters has a more complex structure than traditionally assumed. Objective: To clarify the layer-specific microanatomy of the anal sphincter region and reassess how the fistula pathways described by the Parks classification correspond to actual histological structures. Design: Descriptive histological analysis of cadaveric specimens. Settings: Department of Clinical Anatomy at Institute of Science Tokyo. Patients: Eleven adult, human donors who had consented to whole-body donation. Interventions: Tissue blocks containing the lateral anal canal were collected and processed for histological and immunohistochemical analyses in the transverse and coronal planes. Main Outcome Measures: Identification of the sphincter muscles, longitudinal muscles, levator ani muscles, interbundle gaps, and connective tissue compartments as well as their spatial relationships. Results: The longitudinal muscles exhibited a mosaic structure with dense and loose regions. The dense region ended near the mid-height of the internal sphincter, whereas the loose region expanded inferiorly to form a wide compartment of sparse fibers and loose connective tissue. From this compartment, the loose fibers branched laterally and inferiorly, passing through natural gaps within the external sphincter muscle. Two overlapping layers of the levator ani muscle were also identified. The loose compartment beneath the longitudinal muscle corresponded to the site identified by Parks classification as the origin of abscess formation. Limitations: Specimens were obtained from formalin-fixed cadavers of elderly donors, only the lateral anal canal wall was assessed, and correlations with clinical fistulas were inferential. Conclusions: The region between the sphincter muscles is not a simple plane, and instead, a heterogeneous compartment; its structural features provide a basis for the initiation and propagation patterns described in the Parks classification of anal fistulas.